This invention relates to the preparation of trimethine dyestuffs by reacting a mixture of methylene-substituted indoline compounds with acetic anhydride and formic acid.
The preparation of trimethine dyestuffs is well known. For example, British Patent 232,740 discloses the reaction of certain cyclic ammonium salts having reactive .alpha.- or .gamma.-methyl groups (including .alpha.-methylindoleninium salts) with orthoformic esters or formic acid salts. This method, however, was found to be unsuitable for the preparation of certain substitution products of polymethine dyestuffs. Moreover, even when usable, this method can give rise to various undesirable by-products, such as alkyl acetates and alkyl iodides.
British Patent 328,357 and German Patent 599,560 disclose a method for preparing polymethine dyestuffs that are not accessible by the method of British Patent 232,740. These later references describe the condensation of cyclic ammonium salts having reactive .alpha.- or .gamma.-methyl groups (including the iodide salt of 1,3,3-trimethyl-2-methyleneindoline) with formic acid or a salt of formic acid in a large excess of acetic anhydride. This method, however, has significant deficiencies, particularly when using formic acid as the free acid. For example, according to U.S. Pat. No. 4,419,511, the use of acetic anhydride can partially acetylate the 2-methyleneindoline. In addition, the formation of acetic formic anhydride, a highly reactive mixed anhydride, is believed to play a role in the condensation reaction. Although alkali metal salts of formic acid react readily with acetic anhydride, the free acid reacts less extensively. Consequently, the uniform preparation of stable dyestuff solutions is not obtained when using free formic acid.
One approach intended to avoid the disadvantages of the previously described methods is disclosed, for example, in French Patent 775,578. In this method, a 2-methyleneindoline (e.g., formula (A) in which R is hydrogen) reacts with the corresponding aldehyde (e.g., formula (A) in which R is --CH.dbd.O) under dehydrating conditions, such as provided by using phosphorus oxychloride in benzene or even hot anhydrous pyridine. ##STR3## An obvious disadvantage of this method, however, is the additional preparative steps required to obtain the aldehydes.
The method of U.S. Pat. No. 4,419,511 was intended to avoid the disadvantages of the previously described methods by using alkali metal salts of formic acid in a mixture of acetic acid and acetic anhydride. The presence of sodium ions in the resultant dyestuff, however, apparently affects the stability of concentrated solutions. For example, Example 2 of the reference teaches that acetic acid, ethylene glycol, and an emulsifier based on ethylene oxide and oleyl alcohol must be added to prevent the precipitation of the dyestuff from a concentrated solution. Alternatively, the dyestuffs could be isolated as a solid and then redissolved in the absence of salts. The desirability of avoiding the use of salts is apparent.
An object of the present invention, therefore, was the preparation of trimethine dyestuff concentrates without the disadvantages of the previous methods. -t has surprisingly been found that the condensation of concentrated solutions of 2-methyleneindoline and formic acid in acetic anhydride provides an efficient method for preparing trimethine dyestuffs that are stable in solution even as concentrates.